adds support to externalDocs per route when using OAS 2.0 and updates FastifySchema interface

[riquenunes]

This pull request adds the possibility of specifying an externalDocs object per route when using OAS 2.0 (as described here) and also exposes this option in the FastifySchema interface.

This was already supported by the library when using OAS 3.0 (as seen here), but was not exposed in the FastifySchema interface and was also not available when using OAS 2.0.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm

[coveralls]

Pull Request Test Coverage Report for Build 1715756395

• 0 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 1672154250:	0.0%
Covered Lines:	636
Relevant Lines:	636

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 1715756395

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

---

Totals
Change from base Build 1672154250:	0.0%
Covered Lines:	636
Relevant Lines:	636

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💛 - Coveralls

[climba03003]

LGTM.